Positive displacement air pumps include Roots-type blowers, screw-type air pumps, and many other similar devices with parallel lobed rotors. Positive displacement air pumps may include lobed rotors having either straight lobes or lobes with a helical twist. The rotors may be meshingly disposed in parallel, transversely overlapping cylindrical chambers defined by a housing. Each rotor may have four lobes in conventional embodiments, although each rotor may have fewer or more lobes in other embodiments. Spaces between adjacent unmeshed lobes of each rotor may transfer volumes of compressible fluid (e.g., air) from an inlet port to an outlet port opening, with or without mechanical compression of the fluid in each space prior to exposure of the transfer volumes to the outlet port opening. The ends of the unmeshed lobes of each rotor may be closely spaced from the inner surfaces of the cylindrical chambers to effect a sealing cooperation therebetween. As the rotor lobes move out of mesh, air may flow into volumes or spaces defined by adjacent lobes on each rotor. The air in these volumes may be trapped therein at substantially inlet pressure when the meshing lobes of each transfer volume move into a sealing relationship with the inner surfaces of the cylindrical chambers. Timing gears may be used to maintain the meshing lobes in closely spaced, non-contacting relation to form a seal between the inlet port and outlet port opening. The volumes of air may transferred or directly exposed to the outlet port when the lobes move out of sealing relationship with the inner surfaces of the cylindrical chambers.
Conventionally, positive displacement air pumps may be used as superchargers for vehicle engines, wherein the engine provides the mechanical torque input to drive the lobed rotors. The volumes of air transferred to the outlet port may be utilized to provide a pressure “boost” within the intake manifold of the vehicle engine, in a manner that is well known to those of ordinary skill in the art. The power or energy required to transfer a particular volume of air under certain operating conditions may be used in evaluating the efficiency of a positive displacement air pump. To pump the fluid (e.g., air) using a supercharger requires that mechanical energy be placed into the supercharger. The required mechanical energy input is directly related to the various efficiencies (e.g., mechanical, isentropic, etc.) and operating conditions of the supercharger (e.g., mass flow rate, pressure ratio, etc.). For the same operating conditions, if the efficiency is improved, the required mechanical energy input is decreased, thus benefiting efficiency of the overall system that the supercharger is applied to (e.g., an internal combustion engine). An ideal process would be 100% efficient. However, actual compression will operate at an efficiency below this level. The actual compression relative to the ideal process is called isentropic efficiency. The temperature of the air being transferred may increase as the air flows through the supercharger. By improving isentropic efficiency, less excessive heat energy may be put into the fluid (e.g., air) to achieve the desired pressure for the fluid (e.g., air).
Previous attempts have been made to improve the isentropic efficiency of positive displacement air pumps, such as Roots-type blowers, by improving the configuration of the outlet port. For example, the outlet port of a Roots-type blower may be configured as disclosed and illustrated in U.S. Pat. No. 5,527,168, which is hereby incorporated by reference in its entirety. As technological improvements have been made to supercharger rotor geometry (including, for example, the degree of helical twist), the fluid velocity has been shifted more towards the axial direction, as opposed to the radial direction. However, current parallel shaft supercharger outlet port geometry may continue to account mainly for radial outlet airflow, rather than significantly addressing the axial flow component of the fluid velocity.
It may be desirable to optimize flow geometry at the outlet end of the supercharger to better account for both the axial and radial fluid velocity, while still maintaining the conventional and/or standard features of a supercharger, such as an axial inlet direction and a radial outlet port direction. As supercharger speed increases, the axial velocity component may also increase and may require a more drastic velocity change as it exits the outlet port of a conventional supercharger design. In particular, all axial velocity vectors may be required to be converted into radial velocity vectors, thereby increasing the work that must be performed on the fluid.